Piezoelectric device



Jan. 21, 1947. ,1. E. SHOMER PIEZOELEC'IRIC DEVICE Filed Feb. 23, 1942 q zllzl 1/ EU l INVENTOR. JOHN E. SHOMED Patented Jan. 21, 1947 was PIEZOELECTRIC DEVICE John E. Shomer, Cleveland, Ohio, assignor to The Brush Development Company, Cleveland, Ohio,

a corporation of Ohio Application February 23, 1942, Serial No. 432,020

24 Claims.

This invention relates to piezoelectric devices for the conversion of mechanical into electrical energy or vice versa, and particularly to such devices wherein a flexible crystal element acts as a diaphragm to co-act with the surrounding medium. Devices of the general type to which this invention pertains are exemplified in United States Letters Patent 2,105,010 and are commercially known as Sound Cells. It is to be understood, however, that the term sound as used in this specification is intended to have a very broad connotation. That is to say, it includes vibratory movement in any fluid or gaseous medium and also embraces static pressures as well.

It is an object of the invention to provide a simple and rugged Sound Cell and one that may easily be moisture-proofed.

Another object of the invention is to provide an improved Sound Cell that shall have non-directional response, insensitiveness to mechanical shock, relatively small size, shall have light weight and be simple in construction.

Another object of the invention is to provide a microphone of the Sound Cell type wherein the piezoelectric crystal section is so suspended that its operational resonance is substantially the same as the free resonance thereof.

Another object of the invention is to provide a piezoelectric transducer wherein the response characteristic curve shall be reasonably flat over a wider range of sound frequencies than that obtainable with analogous devices heretofore known.

Another and more specific object of the invention is to provide a Sound Cell wherein only a single Bimorph element is required for nondirectional response.

The foregoing objects and other objects ancillary thereto, in a preferred embodiment of the invention are attained by mounting a piezoelectric crystal Bimorph element of the twister type within a housing so constructed that sound waves impinging on the front of the cell are given access to one pair of diagonally opposite corners or quadrants of the crystal only, while sound waves arriving at the rear face of the cell are permitted to impinge only upon the other two corners or quadrants of the crystal element. More specifically, the Bimorph is supported along nodal lines thereof and between two housing elements or cover plates, each of which is provided with two windows giving sound access respectively, to the pairs of diagonally opposite corners or quadrants of the crystal element with the result that sound pressures additively cause the element to flex around the nodal lines and to develop potentials representative thereof. Y

The supporting means may be constituted by two gaskets, each of which has four windows cut through it, the bars between the openings contacting opposite faces of th crystal element along the nodal lines only, whereby free movement of the corners of the Bimorph is permitted. Sealing means are also provided for preventing sound from reaching the rear surfaces of the active corners of the crystal, the supporting gaskets serving to prevent any two adjacent corners of the Bimorph, as seen from the front or rear of the cell, from being simultaneously exposed to sound pressures acting in the same direction with respect to the plane of the crystal.

The novel features of this invention are set forth with particularity in the appended claims. The invention itself, however, both to its organization and its method of operation, together with further objects and advantages thereotwill best be understood from a consideration of the following description of certain specific embodiments taken in connection with the accompanying drawing in which:

Fig. 1 is a front view in perspective of a preferred embodiment of the invention;

Fig. 2 is a vertical sectional view taken along the line 22 in Fig. 1;

Fig. 3 is an elevational view in perspective of the crystal-supporting gaskets, showing a Bimorph in between them.

Fig. 4 is a view in perspective of an interior element of the device shown in Fig. 1;

Fig. 5 is a vertical sectional view of an alternative embodiment of the invention;

Fig. 6 is a view in perspective of the crystal supporting and damping element embodied in the device exemplified by Fig. 5 and showing a crystal section in place;

Fig. 7 is a view in perspective exemplifying a crystal supporting gasket of the type utilized when a bender crystal is employed;

Fig. 8 is a view in perspective of a Bimorph crystal unit of the twister type, and

Fig. 9 is a View in perspective of a Sound Cell comprising a, piezoelectric element of the bender type.

In all figures of the drawing, identical elements are similarly designated.

Referring now to Fig. l of the drawing, the improved Sound Cell is constituted by a front plate I and a back plate 3, each plate being provided with a, pair of openings 5'o' for permitting sound to impinge directly upon the quadrant areas 6-5 of a Bimorph assembly I of the twister type that is supported between them. Such an assembly i shown in Fig. 8. The plates I and 3 may be of any suitable material such as hard wood, metal, or plastic. The windows -5 in the plate I are located at the opposite ends of a diagonal line joining two of the corners of the "Bimorph assembly as indicated in Fig. 1. The back plate 3 also has two windows 5-5 in it, permitting sound to impinge only upon the other two quadrants of the Bimorph assembly.

For the purpose of preventing sound pressure leakage, so to speak, from each of the active quadrant surfaces to an adjacent quadrant surface on the same side of the crystal element, the element is clamped between two gaskets 9-9, as indicated in Fig. 3 of the drawing. Any other suitable means for isolating the quadrants may be substituted for the gaskets without departing from the spirit of the invention. Each gasket has four windows II in it opposite, respectively, to the corners of the crystal assembly and the bars l3 between the windows contact opposite sides of the crystal assembly along nodal lines.

If gaskets are utilized, as shown, they may be of any suitable material, such as rubber or the like, adapted to make such intimate contact with the'crystal element that the fluid medium, in which the device operates, is prevented from leaking past the junctions. Cement may be used between the askets and the crystal element, or element assembly, if desired.

For the purpose of preventing sound pressure from reaching the rear face of each corner of the crystal as seen through the window corresponding thereto, the crystal assembly may be surrounded by a collar I5 of soft material such as rubber or Viscoloid as shown in Figs. 5 and 6.

Alternatively, as shown in Figs. 2 and 4, the Bimorph may be placed within a relatively stiff frame [1, the depth of which equals the crystal thickness and a sheet IQ of paper, metal foil, or the like may be cemented to one or both faces of the crystal and to the adjacent surface of the frame. If the frame and the paper are utilized, it is advantageous to provide the frame with small inward projections or lug 2i for the purpose of centralizing the crystal, and also to provide a certain amount of compliance adjacent to the crystal corners by impressing corrugations 23 in the surface of the paper thereat. It is to be clearly understood that the paper sheets are not diaphragms in the conventional sense of the word; in effect, sound pressures are applied directly to the crystal.

Although omitted from the drawing, it is to be understood that the finished cell embodies means, such as a plurality of bolts, an encircling band, or the like, for holding the elements in spaced relation and for exerting sumcient pressure upon the gaskets and crystal carrying frame to effectively acoustically isolate the corners of the crystal from each other as well as to seal the edges of the device.

The sub-assembly shown in Fig. 4 may be very thoroughly waterproofed by any suitable method provided that the coverings I9 are not materially stiffened at the crystal corners by the waterproofing process.

Although the frame I! of Fig. 4 has been omitted from Fig. 3 for the sake of simplicity, it is to be under-stood that the spacing lugs thereon correspond in position and width to the cross-bars in the resilient gaskets.

It is also to be understood that many of the dimensions of the drawing have been exaggerated for purpose of explanation, and that they may be materially departed from in constructing an actual device in accordance with the invention.

In operation, sound impinges simultaneously upon the exposed pairs of corners as indicated by the arrows A, B, C, and D of Fig. 8 and is prevented by the coverings I9 of Fig. 4, or the encircling collar l5 shown in Figs. 5 and 6, from reaching the rear of any corner. The gaskets, 9-9, in cooperation with the unperforated surface areas of the apertured plates l and 3 prevent the application of sound pressures simultaneously to adjacent corners of the Bimorph," in the same direction with respect to the plane of the crystal element. The Bimorph, therefore, is caused to flex along nodal lines corresponding in location to the cross-bars of the supporting gaskets 9-9 with the result that alternating potentials appear between the surface electrodes 25 and the central electrode 2?. Inasmuch as all dimensions of the cell may be made small in comparison with the wave length of the highest frequency at which it is to be operative, the device is substantially non-directional.

If it is desired to utilize an element of the bender type instead of the twister type, the gaskets 9-9 may take the general form of an X as shown in Fig. '7 of the drawing, and sound is permitted to impinge on the crystal element through oppositely positioned pairs of openings adjacent the edges instead of adjacent the corners thereof as is the case with the device exemplified by the other figures of the drawing. The appearance of a Sound Cell comprising a bender element is exemplified by Fig. 9.

In a preferred embodiment of the invention, exemplified by Fig. 2, the windows in the cover plates are preferably filled with appropriate damping material '2929, the said material being spaced away from the corresponding corner of the crystal a short distance of the order of .01 of an inch. An appropriate damping material is of the sintered metallic particle type disclosed and claimed in the copending application of A. L. W. Williams et al., Serial No. 541,694, filed June 22, 1944. The material is omitted from Fig. 1 to avoid confusion.

Alternatively, the end plates I and 3 of the sound cell may be entirely constructed of the said metallic damping material rendered impervious to sound intermediate the window-areas by the application of a suitable cement which penetrates the minute passageways therethrough. Such modification has not been illustrated because it is believed to be reasonably obvious to those skilled in the art.

The leads 3l3l from the Bimorph electrodes purposely have been omitted from the several figures with the exception of Fig. 8. They may be brought out in any suitable manner, depending upon the use to which the finished device is to be put. Although a single sound cell utilizing a single Bimorph crystal has been disclosed, it is to be understood that it is possible, if desired, to build up a wall, so to speak, of sound cells, the window openings II lying, respectively, in two planes; or a plurality of single sound cells may be utilized in any desired spatial relation for any given purpose such as to obtain a greater output or to secure directivity. In such event the proper connecting of the terminals is a matter of electrical design well within the knowledge of one skilled in the art, and need not be illustrated.

l. 15... s ,l --i-. Also either a single sound cell :or a plurality .ofsthe improved sound cells may be utilized as sources. of sound or ofvibratory phenomena in any. given medium. provided the electrodes are properly supplied with operating potentials.

Among the advantages of the improved sound cell may be mentioned the fact that the nodal linesupporting gaskets E -9-permit the crystal to. resonate at substantiallysits free frequency. .The. showing .of nodalsupports inthe Piersol 'patent,.,1,930,536 is, of course,-known. The

patent inquestion, however, relates to a quartz crystalwand instead of the crystal beingleft free to ..vibrate at .its natural: frequency, the patent has. for ,.its...object the obtaining of much lower than natural frequencies by forcible driving. Such result is attained by properly electroding the. corners of I the crystal and cross-connecting themtothe input circuit of an oscillator. In the improved sound cell, however,- the crystal is purposelyileft as nearly free as possible and sound pressures .are. applied to the corners thereof, which pressures act additively in producing electrical potentials.

Another. of the important advantages of the construction resides in the ease with which the. crystal sub-assembly may be waterproofed andv protected against climatic changes either through use-f the encircling collar l exemplifled .by Figs. 5 and 6, or by the drum construction shown in Fig. 4.

.Further modifications of the invention will be apparent to those. skilled in the art to which it pertains. The invention, therefore, is not to be limited except insofar as is necessitated by the rant lying in one face of the element, directly coupled to the fluid medium, and means for preventing the simultaneous coupling of the sur-- face area of said quadrant lying in the other face of the crystal element to said fluid medium.

2. A transducer planar piezoelectric element having a mode of free vibration characterized by the simultaneous displacement of diametrically opposite quadrants .in the same direction with respect to the plane thereof while adjacent quadrants are dis-- placed in opposite directions with respect to saidplane, said piezoelectric element being disposed in a fluid medium with the surface-areas of a pair of diametrically disposed quadrants, lying in one face of the element, directly coupled to the fluid medium, and means for preventing the simultaneous coupling of the surface areas of said quadrants lying in the other face of the crystal element to said fluid medium.

3. A transducer as set forth in claim 3; f rther characterized in that acoustic dampingmeans is interposed in the path between the fluid medium and the said directly coupled area.

4. A transducer as set forth in claim 2, further characterized in that acoustic dampin means is inter osed in the path between the fluid comprising a substantially medium and the said directly coupled surface areas 1". j

5; In -atransducer-- adapted to utilization 'in a fluid medium, a; frame provided with a plurality of lugs extending inwardly from the walls thereof, the depth of the frame" being substantially equivalent to the thickness of a piezoelectric element havinggiven dimensions, a plurality of planar sealing elements of flexible waterproof material, one for each face of th'e"frame,-the said elements being hermetically aflixed, respectively, along their'peripheriesto opposite faces of the frame to constitutea closed container, and a piezoelectric element of the given dimensions disposed within said container with its opposite major faces in' contact, respectively, with the inner surfaces of the-sealing elements and its edge faces in contact with the inner surfaces of said lugs; and means connected to said frame for preventing the coupling of only a portion of the surfaceof at least one of said major faces to the fluid medium. 1

6. The invention'set forth in claim 5, characterizedin thatthe surfaces ofthe piezoelectric element are *affixed, respectively, to the sealing element's whereby the element is positioned within the container.

'7. The device as set forth in claim 5 further characterized in this: that the said opposite major faces of the said piezoelectric element are connected substantially throughout their entire area to the said sealingelements.

8.Thedevice as set forth in claim 5 further characterized in this: that only a portion of the edge face area of said piezoelectric element is in contact with said plurality of inwardly extending lugs thereby establishing clearance space between the frame and at least a portion of the edge-face area of said piezoelectric element, and that the said planar sealing elements'ar'e shaped for increased flexibility adjacent said clearance space.

9. A transducer comprising a substantially planar piezoelectric element having a mode of free vibration characterized by the simultaneous displacement of diametrically opposite quadrants in the same direction with respect to the plane thereof while adjacent quadrants are displaced in opposite directions with respect to said plane, said piezoelectric element being disposed in a fluid medium'with'the surface area of one quadrant, lying in one face of the element, directly coupled to the fluid medium, and means associated' with said piezoelectric element for preventing the simultaneous coupling of the surface areaof an adjacent quadrant of said one face to said fluid medium.

10. A transducer comprising a, substantially planar piezoelectric element havinga mode of free vibration characterized by the simultaneous displacement of diametrically opposite quadrants in thesame direction with respect to the plane thereof while adjacent quadrants are displaced in opposite directions with respect to said plane, said piezoelectric elementv being disposed in a fluid medium with the surface areas of two diametricallyppposite quadrants of one face thereof directlycoupledlto the fluid medium, and means v associatedwithsaid piezoelectricelement for preventing the simultaneous coupling of the surface ther: characterized-in that acoustic damping means is interposed in the path between the fluid medium and the said .directly coupled surface area of the piezoelectric element.

12. A transducer as set forth in claim 10, further characterized in that acoustic damping means is interposed in the path between the fluid medium and the said directly coupled surface area of the piezoelectric element.

13. A transducer comprising a substantially planar piezoelectric element having a mode of free vibration characterized by the simultaneous displacement of diametrically opposite quadrants in the same direction with respect to the plane thereof while adjacent quadrants are displaced in opposite directions with respect to said plane, said piezoelectric element being disposed in a fluid medium with the surface area of a first pair of diametrically disposed quadrants, lying in one major face of the element, directly coupled to the said fluid medium; and with the surface area of a second pair of diametrically disposed quadrants, lying in the other major face of the element, directly coupled to the said fluid medium; means associated with said one face for preventing the coupling of the surface areas of a third pair of diametrically disposed quadrants to the fluid medium, and means associated with said other major face for preventing the coupling of the surface areas of a fourth pair of diametrically disposed quadrants to the fluid medium, the coupled surface areas of each major face substantially registering with the uncoupled surface areas of the other major face.

14. A transducer comprising, in combination, a substantially planar piezoelectric element comprising a plurality of parallel plates of crystalline material so oriented with respect to the crystallographic axes and connected together that it is adapted to flex under the influence of electrostatic fields, frame means, means for mounting said piezoelectric element in said frame means whereby the element has a mode of substantially free vibration characterized by the simultaneous displacement of diametrically opposite quadrants in the same direction with respect to the plane thereof while adjacent quadrants are displaced in opposite directions with respect to said plane, said piezoelectric element being disposed in a fluid medium with the surface area of one quadrant of one face thereof directly coupled to the fluid medium, and means connected to said frame means for preventing the simultaneous coupling of the surface area of an adjacent quadrant of said one face to said fluid medium.

15. The invention as set forth in claim 14 further characterized by acoustic damping means mounted on said frame means closely adjacent to the surface area of the quadrant of the said one face of the piezoelectric element which is coupled to the fluid medium.

16. A transducer comprising, in combination, a substantially planar piezoelectric element comprising a plurality of parallel plates of crystalline material so oriented with respect to the crystallographic axes and connected together that it is adapted to flex under the influence of electrostatic fields, frame means, means for mounting said piezoelectric element in said frame means whereby the element has a mode of substantially free vibration characterized by the simultaneous displacement of diametrically opposite quadrants in the same direction with respect to the plane thereof while adjacent quadrants are displaced in opposite directions with respect to said plane;

said piezoelectric element being disposed in a fluid medium with the surface areas of two diametrically opposite quadrants of one face thereof directly coupled to the fluid medium, and means connected to said frame means for preventing the simultaneous coupling of surface areas of the two adjacent diametrically opposite quadrants of said face to said fluid medium.

1'7. A transducer comprising a substantially planar piezoelectric element having a mode of free vibration characterized by the simultaneous displacement of two diametrically opposite quadrants in the same direction with respect to the plane thereof while the two adjacent quadrants are displaced in the opposite direction with respect to said plane, frame means connected to said piezoelectric element for supporting said element to permit substantially said mode of free vibration, said piezoelectric element being disposed in a fluid medium with the surface area of one quadrant, lying in one face of the element, coupled to the fluid medium, and acoustic shielding means supported by said frame means for preventing the simultaneous couplin of the surface area of said one quadrant lying in the other face of said crystal element to the surrounding fluid medium.

18. A transducer comprising a substantially planar piezoelectric element having a mode of free vibration characterized by the simultaneous displacement of two diametrically opposite quadrants in the same direction. with respect to the plane thereof while the two adjacent quadrants are displaced in the opposite direction with respect to said plane, frame means connected to said piezoelectric element for supporting said element to permit substantially said mode of free vibration, said piezoelectric element being disposed in a fluid medium with the surface area of two diametrically opposite quadrants, lying in one face of the element, coupled to the fluid medium, and acoustic shielding means supported by said frame means for preventing the simultaneous coupling of the surface area of the said two diametrically opposite quadrants, lying in the other face of said element, to the surrounding fluid medium,

19. A transducer comprising a substantially planar piezoelectric element having a mode of free vibration characterized by the simultaneous displacement of two diametrically opposite quadrants in the same direction with respect to the plane thereof while the two adjacent diametrically disposed quadrants are displaced in the opposite direction with respect to said plane, frame means connected to said piezoelectric element for supporting said element to permit substantially said mode of free vibration; said piezoelectric element being disposed in a fluid medium with the surface area of two diametrically disposed quadrants, lying in one face of said element, coupled to the fluid medium, and with the surface area of two adjacent diametricall disposed quadrants, lying in the other face of said element, coupled to the fluid medium; and acoustic shielding means supported by said frame means for preventing the simultaneous coupling of the surface area of the two diametrically disposed quadrants, lying in said other face, to the fluid medium, and for preventing the simultaneo'us coupling of the surface area of the two adjacent diametrically disposed quadrants, lying in said one face, to the fluid medium.

20. A transducer as set forth in claim 13 further characterized in that acoustic damping means is interposed in the path between the fluid 9 medium and the said directly coupled surface areas.

21. The transducer as set forth in claim 16 further characterized by acoustic damping means mounted on said frame means closely adjacent to the surface areas of said piezoelectric element which are coupled to the fluid medium.

22. The transducer as set forth in claim 17 further characterized by acoustic damping means mounted on said frame means closely adjacent to the surface area of said piezoelectric element which is coupled to the fluid medium.

10 23. The transducer as set forth in claim 18 further characterized by acoustic damping means .mounted on said frame means closely adjacent to the surface areas of said piezoelectric element which are coupled to the fluid medium.

24. The transducer as set forth in claim 19 further characterized by acoustic damping means mounted on said frame means closely adjacent to the surface areas of said piezoelectric element 10 which are coupled to the fluid medium.

JOHN E. SHOME-R. 

